The Afghanistan veteran challenged the Prime Minister as he defended savings of 7.5 per cent. Royal Navy lieutenant commander Kris Ward said: "I am a Harrier pilot and I have flown 140-odd missions in Afghanistan, and I am now potentially facing unemployment. How am I supposed to feel about that, please, sir?"

The 80-strong fleet of Harrier jets will be decommissioned from early next year.

Speaking after a question and answer session at Permanent Joint Headquarters in north-west London, Lt-Cdr Ward, 37, added: "I understand that cuts have to be made, but I am not sure that these are the right cuts."

The cutbacks will mean the loss of more than 40,000 defence jobs. Mr Cameron said: "We do have to make decisions for the future and there have been long discussions about this in the National Security Council."

He thanked Lt-Cdr Ward for "everything", but added: "I have listened to all the military advice, and the military advice is pretty clear that when we have to make difficult decisions, it is right to keep the Typhoon as our principal ground attack aircraft, working in Afghanistan at the moment, and it is right to retire the Harrier."

The Prime Minister admitted that "difficult" decisions had been made in the strategic defence and security review as he addressed staff at the operations headquarters. But he insisted Britain would remain a "front rank military power".

Lt Cdr Ward's father, retired Commander Nigel "Sharkey" Ward defended the Harrier jet today on which he has written a book.

He told the BBC: "The Navy Harrier ensured success in the Falkands.

"Our troops on the ground of course achieved that final success but without the Harrier it could not have happened."

In today's landmark reshaping of the armed forces, civilian workers will bear the brunt with 25,000 posts to go out of a total of 40,000, including hundreds at the Ministry of Defence in Whitehall.

The Army will lose 7,000 military posts, the Royal Navy and RAF around 5,000 each. The grim figures on the human cost of the cuts emerged as Defence Secretary Liam Fox took to the airwaves to defend the extraordinary situation of Britain's aircraft carriers not having any UK planes on them for 10 years.

French and US aircraft may even be the first to operate off one of two new British aircraft carriers being built at a cost of £6 billion.

The replacement of the Trident nuclear deterrent is also being delayed, averting a major showdown between the Liberal Democrats and the Conservatives over its future. On Trident, Mr Fox said: "I do not believe that any of the measures that we take will in any way affect the effectiveness of our nuclear deterrent, nor our ability to have a continuous at-sea deterrent."

Britain's special forces are due to get extra funding and there will be a £20 million boost to medical services.

Mr Cameron was today announcing that the Navy's flagship HMS Ark Royal will be scrapped early and the 80-strong fleet of Harrier jets will be decommissioned from early next year.

Britain's remaining aircraft carrier HMS Illustrious will be used as a platform for helicopters but is due to go out of service in 2014, leaving the UK with a maritime "capability gap".

The new carrier HMS Queen Elizabeth was due to enter service in 2016 but this date may be delayed. The second carrier being built, HMS Prince of Wales, is due to be ready in 2019.

But one of them may be then mothballed and could then be sold off, possibly to India. The remaining aircraft carrier will be fitted with an electromagnetic system of catapults to launch planes and drag wires for landings.

Mr Fox insisted today it was not unprecedented for Britain to have aircraft carriers without jets. Tornado and Typhoon jets would allow Britain to project "air power".

Asked about the situation where French rather than British planes may fly from one of the new carriers, he added: "You need to be looking not to the end of the decade but to the 35 to 40 years of life of the carriers, and to have inter-operability with our allies seems to me to be a priority in that period if we are to have effective alliances."

Experts have criticised the military overhaul as "indecision-led" and "eccentric".

The Lib-Con coalition is blaming the shake-up on £38 billion of military over-commitments inherited from Labour.

The £37 billion defence budget is being cut by 7.5 per cent but MoD insiders stress that this equates to around 16 per cent once the nine per cent because of the overspend from the previous government is taken into account.

An order for 25 Chinook helicopters is to be cut to 12. Britain's 20,000 troops in Germany will be withdrawn by 2020, with 10,000 out by 2015.

Mr Cameron was set to announce that Britain's military spending will not fall below two per cent of Gross Domestic Product, the minimum expected of Nato members.

Unions reacted angrily to the looming job cuts. Kevin Coyne, Unite national officer for the MoD, said: "MoD civilian staff in London play a vital logistical role in maintaining national security at a time of heightened alert."

Mr Cameron spoke to President Obama last night before unveiling the strategic defence and security review. Last week, US Secretary of State Hillary Clinton said Washington was "worried" by the scale of UK defence cuts.

But a Downing Street spokesman said the Prime Minister had promised President Obama that the UK would "remain a first-rate military power and a robust ally of the United States".

It would "continue to work closely with the US on the full range of current security priorities," he added.

The last strategic defence review in 1998 took more than a year, while this one has been carried out in five months, leading to accusations that the government has rushed the process.

It has been undertaken at the same time as the comprehensive spending review to be published tomorrow which is expected to see huge cuts to departmental spending across Whitehall.

Rear Admiral Terry Loughran, who was at the helm from 1993 to 1994 during the Bosnian conflict, said the decision to have no flight capability on aircraft carriers for up to 10 years would lead to a massive loss of skills.

He said the decision to adapt the new ships to host conventional aircraft rather than those with hover capabilities would lead to greater long-term costs. He added: "It is the scrapping of the Harriers that gives me the greatest concern and highlights that the review is far from strategic."

THE NAVY

Two new aircraft carriers will be built at a cost of £6 billion but about 5,000 jobs will go. HMS Ark Royal will be retired early and the fleet of Harrier aircraft decommissioned. The second carrier, HMS Illustrious, will be used as a helicopter platform but is due to be retired in 2014. One of the two new aircraft carriers, HMS Queen Elizabeth and HMS Prince of Wales, is expected to be mothballed in 2019 and possibly sold off. The other would be able to take conventional jets rather than vertical take-off aircraft. Four frigates will be axed as thesurface fleet is cut from 24 to 19. An order for seven Astute Class hunter-killer submarines will go ahead and Trident is delayed.

THE RAF

The air force will lose some 5,000 military posts and at least two bases. RAF Kinloss in Scotland is seen as the most vulnerable. The new Nimrod MRA4 fleet is expected to be scrapped. The Tornado fleet will escape the immediate axe but bephased out with the introduction of new Typhoon and Joint Strike Fighter jets. But the number of Joint Strike Fighter planes is due to be cut from about 130 to 40. RAF bases could be used for soldiers returning from Germany.

THE ARMY

The Army is due to lose some 7,000 soldiers, more than 100 tanks and 200 armoured vehicles. The job losses are far lower than the 20,000 originally feared. Twenty thousand troops in Germany will be brought home by 2020. David Cameron has guaranteed that operations in Afghanistan will notbe hit by the cuts. General Sir Peter Wall, the Chief of the General Staff, intervened at the 11th hour to oppose changes to training which he felt would affect the Afghan campaign. Special forces will get extra funding to buy cutting-edge communications technology and weapons.